Arrangement for synchronizing access points in WLAN using direct-sequence spread spectrum signaling

ABSTRACT

An arrangement for providing synchronization between a number of overlapping area access points within a wireless LAN utilizes a Power over Ethernet (PoE) cable connection to transmit synchronization signals from a centralized hub/switch to each of the access points connected to the cable. The synchronization signal takes the form of a direct sequence spread spectrum (DS-SS) signal that is coupled onto the twisted pair used to provide the low voltage PoE signal to various powered devices. At each access point, a filter is used to remove the synchronization signal from the PoE transport, allowing for the group of access points to share a common beacon signal and allow for frequency re-use among the access points. The DS-SS signal is preferable created by a combination of a baseband signal and a set of separate pseudo noise (PN) sequences defining a precision phase (“pilot tone”) signal, a time mark (superframe/beacon) signal, and a time-of-day (real-time clock) signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.60/796,177, filed Apr. 29, 2006.

TECHNICAL FIELD

The present invention relates to an arrangement for synchronizingtransmission between a plurality of access points (APs) in a wirelesslocal area network (WLAN) and, more particularly, to an arrangement thatutilizes direct-sequence spread spectrum (DS-SS) signaling via a “Powerover Ethernet” (PoE) wired connection.

BACKGROUND OF THE INVENTION

A wireless local area network (WLAN) is generally implemented to providelocal connectivity between a wired network and one or more mobilecomputing devices. In a typical wireless network, all of the computingdevices within the network broadcast their information to one anotherusing radio frequency (RF) communications. WLANs are based on the IEEE802.11 standard, which designates a wireless-Ethernet specificationusing a variety of modulation techniques at frequencies in the 2.4 GHzand 5 GHz license-free frequency bands, although other bands might beused with the same basic framework.

The IEEE 802.11 standard (generally referred to as “Wireless Fidelity”,or Wi-Fi) enables wireless communications with throughput rates up to 54Mbps. Wi-Fi is essentially a certification that a manufacturer's productis interoperable with the IEEE 802.11 standard. For example, equipmentcarrying the “Wi-Fi” logo is certified to be interoperable with othercards that operate in peer-to-peer mode, but Wi-Fi usually incorporatesat least one access point, or edge device. Most access points have anintegrated Ethernet controller to connect to an existing wired-Ethernetnetwork. A Wi-Fi wireless transceiver connects users via the accesspoint to the rest of the LAN. The majority of Wi-Fi wirelesstransceivers available are in Personal Computer Memory CardInternational Association (PCMCIA) card form, particularly for laptop,palmtop, and other portable computers. However, Wi-Fi transceivers canalso be implemented through an Industry Standard Architecture (ISA) slotor Peripheral Component Interconnect (PCI) slot in a desktop computer,mini-PCI cards embedded in a laptop computer, a Universal Serial Bus(USB), or can be fully integrated within a handheld device.

Modern WLANs composed of more than one access point with overlappedcells require a frequency re-use pattern to minimize interference.However, available frequencies may be too few to allow for a sufficientamount of co-channel interference reduction. Recently, newquality-of-service (QoS) protocols allow for APs to share “time” as wellas “frequency”. However, synchronization between APs is required forsuch time division multiplexing schemes. One prior art arrangement forproviding synchronization between access points is discussed in U.S.Pat. No. 6,917,804, issued to T. Takayama et al. on Jul. 12, 2005. Inthis case, one access point is defined as the “master” access point andperiodically transmits a “master beacon” to all remaining access pointsin the WLAN. Such an arrangement, however requires each access point tobe equipped with additional receiving and synchronization components.

A need remains, therefore, for an arrangement that allows forsynchronization of access points with little additional overhead, whilemaintaining the reliability of a wired connection.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to an arrangement for synchronizingtransmission between a plurality of access points (APs) in a wirelesslocal area network (WLAN) and, more particularly, to an arrangement thatutilizes direct-sequence spread spectrum (DS-SS) signaling via a “Powerover Ethernet” (PoE) wired connection.

In accordance with the present invention, DS-SS signaling is used toprovide a wideband, noise-like waveform for communicating three timingsignals: “precision phase”, “time mark” and “time-of-day” over PoE pairof a “Category 5” (hereinafter “CAT5”) Ethernet cable. In particular,the spread spectrum signals are launched and recovered on a separate PoEpair using a simple power-separation filter. The synchronization signalsare used, for example, to align beacons for the access points, allowingthem to time share the same channel without interference.

Other and further aspects of the present invention will become apparentduring the course of the following discussion and by reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 illustrates an exemplary prior art wireless LAN arrangementincluding a plurality of overlapping coverage access points (APs);

FIG. 2 illustrates an exemplary prior art Power over Ethernet (PoE)arrangement for providing both data and low power signals from a powersourcing equipment (PSE) module to a powered device (PD) end station;

FIG. 3 contains a diagram of an exemplary embodiment of the presentinvention, where the PoE Ethernet cable connection is used to providedirect-sequence spread spectrum (DS-SS) synchronization signals from ahub connection to an exemplary access point over a “spare” cable pair;

FIG. 4 illustrates an exemplary embodiment of a power separation filterutilized to combine a power signal (I) and DS-SS sync signal (S) ontothe same cable pair within a CAT5 Ethernet cable;

FIG. 5 illustrates an alternative embodiment of the present inventionwhere the pair of cables carrying the data signal are also used to carrythe synchronization signal;

FIG. 6 illustrates, in simplified block diagram form, a plurality ofaccess points coupled to a single hub location via an Ethernet cablethat supports the transmission of the same synchronization signal toeach access point, thus allowing for frequency re-use in accordance withthe present invention;

FIG. 7 illustrates an exemplary synchronization generator module for usein generating an exemplary DS-SS synchronization signal to betransmitted over the Ethernet cable; and

FIG. 8 illustrates an exemplary demodulator for use in an AP to recoverthe various synchronization pulses from the propagating synchronizationsignal.

DETAILED DESCRIPTION

The present invention is based on leveraging the communicationcapability provided by the emerging Power over Ethernet (PoE) technologyto “piggyback” synchronization signals onto the power signals, therebydistributing the synchronization signals to each of the APs within thewireless LAN without requiring additional overhead, and allowing forgreater frequency re-use between adjacent, overlapping cells.

FIG. 1 is a diagram of a conventional prior art wireless LAN system 10comprising a plurality of access points (APs) 12 coupled to acommunication switch (or hub) 14 via a conventional Ethernet CAT5 cable16. The radio frequency coverage for each AP 12 is illustrated as ashaded cell region 18, where the overlap between adjacent cells 18 isillustrated as cross-hatched areas 20. As mentioned above, anoverlapping coverage condition may produce too much interference forsome wireless devices in overlap regions 20, illustrated as wirelessdevices 22 in FIG. 1. That is, unless there is a frequency re-use plan,or a radio resource time-sharing arrangement within wireless LAN 10,devices 22 will experience transmission errors.

FIG. 2 illustrates a conventional prior art PoE arrangement that isutilized to provide power signal input over Ethernet cable 16 between apower sourcing equipment (PSE) module 30 and a powered device (PD) 32.While the intricacies of the power distribution are not germane to thesubject matter of the present invention, an overall understanding of thePoE system is helpful. Referring to FIG. 2, Ethernet cable 16 comprisesin this embodiment a CAT5 cable which includes four twisted pairs ofcopper signal paths. A first pair (designated 16-1) is used to providedata and power signals from PSE 30 to PD 32. A second pair (designated16-2) is used to provide the return data signals from PD 32 to PSE 30.The remaining pairs (16-3 and 16-4) have previously been reserved as“spare” communication lines in the event of a failure of either primarypair. Alternatively, and particularly for high speed applications, cablepairs 16-3 and 16-4 may be used as a second data line. When implementingPoE, the power signal is transmitted over data pair 16-1, 16-2 (or“spare” pair 16-3, 16-4) in a “common mode” powering arrangement. If thespare pairs are used for powering, the individual wires in a pair areshorted together at both ends, allowing both conductors to share thecurrent being supplied.

With this understanding of the capability of distributing low voltagepower signals over Ethernet cable, the subject matter of the presentinvention may be explained as follows. Referring to FIG. 3, an exemplarywireless LAN synchronization arrangement for overlapping APs isillustrated as comprising a hub 14 and an exemplary AP 12 connected viaEthernet cable 16. For the sake of clarity, only a single AP 12 is shownin this view. In accordance with the present invention, hub 14 comprisesa PSE 30, similar to the arrangement of FIG. 2. Hub 14 further comprisesan Ethernet line driver 31 for supplying an input data signal (E) topair 16-1 of cable 16, and an Ethernet line receiver 33 for recoveringreceived data from cable pair 16-2 of CAT5 cable 16.

In accordance with the present invention, hub 14 further comprises asynchronization generator module 40 that is utilized to generate adirect-sequence spread spectrum (DS-SS) synchronization signal S that iscoupled through a first power separation filter 42 and transmitted to AP12 over cable 16. It is to be understood that sync signal S may begenerated and transmitted to AP 12 whether or not power I is also beingtransmitted to AP 12 via PoE over cable pair 16-3. Advantageously, thecreation of a DS-SS signal for sync signal S allows for a relatively lowpower, broadband signal to be formed that will not interfere with thedata also being transmitted over cable pair 16-1 (or with any powersignal than may be present along cable pair 16-3).

At the receive side within AP 12, a second power separation filter 44 isused to separate sync signal S from any power signal I that may bepresent along pair 16-3. Synchronization signal S is thereafter appliedas an input to a DS-SS synchronization demodulator 46 to recover thespecific sync signal S, as identified below, that is used to maintainoverall synchronization between the plurality of separate access points.The power signal I (if present) is then applied as an input to a powersinking device (PD) 32, as shown in FIG. 3.

In a preferred embodiment of the synchronizing method discussed hereinand as particularly illustrated in FIG. 3, spare pairs 16-3, 16-4 ofCAT5 cable 16 are employed to transmit the synchronization signals (andpower signals) between hub 14 and a plurality of APs 12. In this case,first power-separation filter 42 is interposed between PSE 30,synchronization generator module 40, and CAT5 cable 16 at hub location14. It is to be understood that if a plurality of CAT5 cables areassociated with hub 14, a single power supply, power separation filter,and synchronizing generator can be shared among the plurality of cablesto provide the desired synchronization signal.

The function of first power separation filter 42 is to prevent thehigh-frequency DS-SS sync signal S generated by module 40 from beingattenuated by the low resistance of the power supply/powering sourcingdevice 30, thus more efficiently coupling the sync signal S onto sparecable pair 16-3. FIG. 4 illustrates an exemplary arrangement of firstpower separation filter 42, sync generator module 40 and cable pairs16-3 and 16-4. An LC circuit arrangement 43 is used to maintainseparation between the DC power signals Idc) and the relatively highfrequency sync signal S. In particular, as a result of itshigh-frequency content, sync signal S propagates through the cable,which behaves as a balanced transmission line, in a relatively unimpededfashion. This minimizes coupling into the data pairs and easescompliance with FCC emissions rules. Second power separation filter 44within each AP 12 is similar in form to first power separation filter 42and functions to isolate sync signal S at the output thereof.

In an alternative embodiment of the present invention, as illustrated inFIG. 5, sync signal S is directly applied to cables pairs 16-1 and 16-2in combination with data signal E. Since DS-SS sync signal S isnoise-like and spread so that its power is distributed over a widefrequency range wide, there will be a minimal amount of interferencewith Ethernet data transmission (particularly the carrier-sensefunction). However, this alternative embodiment requires that thesynchronization and data signals be combined before being fed to aline-coupling transformer at the hub. Additionally, a power signal I mayalso be transmitted over cable pair 16-1. As shown, power signal I iscoupled through a center-tapped transformer 34 onto data pair 16-1 alongwith the DS-SS signal and the data signal. Upon reaching AP 12, the lowvoltage DC current I is returned via another center-tapped transformer36, with the current passing through PD 32 as shown. The DS-SS syncsignal S is removed from the Ethernet data signal E by DS-SSsynchronization demodulator 46, in a manner similar to that discussedabove in association with the embodiment of FIG. 3. Aside from smalldegradation of the Ethernet signal-to-noise ratio, this alternativearchitecture also complicates mid-span powering since the pair of cablesmust be broken to impress the synchronization signal via metallic(rather than common mode) current while retaining line balance. This isbest accomplished by turning the mid-span powering tap into an Ethernetrepeater.

FIG. 6 illustrates, in a simplified overview, the arrangement of thepresent invention where sync signal S as created within sync generatormodule 40 of hub 14 is thereafter transmitted along Ethernet cable 16and received by each AP 12 connected to cable 16. As long as each AP 12includes the same demodulator 46, each AP 12 will recover the same syncsignal and be able to maintain synchronous communication within thewireless LAN formed by the plurality of APs 12. In particular, syncsignal S is used to align the superframes (beacons) for each AP 12 inthe network, allowing them to time-share the same frequency channelwithout causing interference. The synchronization also facilitates powersaving within the wireless client devices and provides for improvedhand-off of client devices between APs.

FIG. 7 illustrates an exemplary synchronization generator module 40formed in accordance with the present invention. In a preferredembodiment, a pair of independent pseudo noise (PN) sequences areutilized to form the DS-SS sync signal S. A first PN sequence, denotedPN-1, is used to generate a precision phase signal that is utilized asthe “pilot” tone for the other sync signals. PN-1 may comprise eitherall 0's or all 1's for fast acquisition. A time mark signal TM thatfacilitates the ability to provide the superframe (beacon) timing amongthe APs (as mentioned above) and “time of day” (ToD) coding signal arecombined within a multiplexer 50 and thereafter spread by a second PNsequence (PN-2), for example, as a real-time clock facility for AP 12,providing ranging, delay measurements and the like. Sequence PN-2 is ofthe same length as PN-1, but comprises an equal number of 0's and 1's.Alternatively, a longer PN sequence may be used for PN-2 so as to makethe signal more noise-like.

As shown in FIG. 7, each of these PN sequences is applied as a separateinput to a summer 54, where Ethernet data signal E is applied as a thirdinput to summer 54. Summer 54 adds the synchronization information(pilot, ToD, TM) to the already-existing Ethernet data signal E (similarto adding white noise to a signal) and then transmits the signal viaCAT5 cable 16 to the remote APs. It is to be understood that in variousother embodiments, perhaps only PN-1 is utilized, or a combination ofPN-1 and PN-2, or any other appropriate combination of one or more PNsequences that supply sync pulses to the plurality of separate APs. Theresultant DS-SS sync signal S is generally in the form of a noise-likespectra, low enough to provide negligible crosstalk with the data and/orpower signals propagating along cable 16.

FIG. 8 illustrates an exemplary demodulator 46 used within each AP 12 toextract the various PN sequences from the received signal travelingalong cable 16. As shown, the received signal is demodulated using thesame baseband signal B and a relatively straightforward filteringarrangement is then used to separate the various PN sequences from thesignal. Thus, as long as each AP 12 recovers the same sync pulses(pilot, TM, ToD), the various units will be able to provide time sharingof the frequency spectrum without the interference problems prevalent inthe prior art.

While the present invention has been described in relation to aparticular preferred embodiment, many variations, equivalents,modifications and other uses will become apparent to those skilled inthe art. Thus, the present invention is intended to be limited only bythe scope of the claims appended hereto.

1. In a wireless local area network (LAN) comprising a plurality ofaccess points interconnected with a hub through an Ethernet cable, eachaccess point providing wireless communication services to one or morewireless client devices in its vicinity, an arrangement for providingsynchronous communication between each access point in the plurality ofaccess points comprising a synchronization generator module located atthe hub for generating a direct sequence spread spectrum (DS-SS)synchronization signal from the combination of a baseband signal and atleast one pseudo noise sequence including timing information forproviding synchronization; and a Power over Ethernet (PoE) couplingarrangement disposed between the synchronization module and the Ethernetcable for coupling the DS-SS synchronization. signal onto said Ethernetcable and transmitting said DS-SS synchronization signal to theplurality of access points.
 2. The arrangement as defined in claim 1wherein the synchronization module generates the DS-SS synchronizationsignal from a combination of the baseband signal and a first pseudonoise sequence defining a precision phase signal, the precision phasesignal utilized as a pilot tone to maintain synchronization between theplurality of access points.
 3. The arrangement as defined in claim 2wherein the DS-SS synchronization signal is generated from a combinationof the baseband signal, the first pseudo noise sequence and a secondpseudo noise sequence defining a time mark signal, the time mark signalutilized as a superframe/beacon synchronization signal between theplurality of access points.
 4. The arrangement as defined in claim 3wherein the DS-SS synchronization signal is generated from a combinationof the baseband signal, the first and second pseudo noise sequences anda third pseudo noise sequence defining a time-of-day signal, thetime-of-day signal used within each access point to provide a real-timeclock signal.
 5. The arrangement as defined in claim 1 wherein theEthernet cable comprises a CAT5 Ethernet cable.
 6. The arrangement asdefined in claim 1 wherein the arrangement further comprises ademodulator/filtering element disposed in association with each accesspoint of the plurality of access points, the demodulating/filteringelement configured to recover the at least one pseudo noise sequencefrom the transmitted DS-SS synchronization signal and extract the timinginformation therefrom.
 7. The arrangement as defined in claim 1 whereinthe synchronization signal is applied to the spare pairs of the CAT5cable according to PoE practice for use of non-data pairs.
 8. Thearrangement as defined in claim 1 wherein the synchronization signal isapplied to the data pairs via a center-tapped transformer according toPoE practice which provides common-mode rejection of the synchronizationsignal by the data receiver/transmitter.